1. Field of the Invention
The invention relates to multiple ratio geared transmissions for use in an automotive vehicle powertrain and to a control strategy for effecting engagement and release of transmission friction torque establishing elements during a ratio change.
2. Background Art
Ratio changes in a geared automatic transmission in an automotive vehicle powertrain are achieved by engaging a so-called on-coming clutch as a so-called off-going clutch is released. The clutches, which may be referred to as transmission friction elements or brakes, establish and disestablish power flow paths from an internal combustion engine to vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of transmission input shaft speed to output shaft speed, is reduced as vehicle speed increases for a given engine throttle setting. This is a ratio upshift. A downshift to achieve a higher speed ratio occurs as the engine throttle setting increases for any given vehicle speed, or when the vehicle speed decreases as the engine throttle setting is relaxed. This is a power-on downshift, or a coasting downshift, respectively.
For purposes of the present disclosure, an upshift or high gear configuration of the transmission with power on will be described as having a high or a higher speed ratio (transmission output speed/transmission input speed) during acceleration of the vehicle. Further, a downshift or low gear configuration of the transmission with power on will be described as having a low or a lower speed ratio during acceleration of the vehicle.
In the case of a synchronous upshift, the on-coming clutch engages to lower both speed ratio and torque ratio, the latter being the ratio of output torque to input torque. The upshift event can be divided into three phases, which may be referred to as a preparatory phase, a torque phase and an inertia phase. For the synchronous upshift, the torque phase is hereafter defined as a time period when the off-going clutch torque is purposely controlled to decrease toward a value of zero or a non-significant level with an intention to disengage it. Simultaneously, during the torque phase, the on-coming clutch torque is purposely controlled to increase from a value of zero or a non-significant level, thereby initiating the on-coming clutch engagement according to a conventional upshift control. The timing of clutch engagement and disengagement results in a momentary simultaneous activation of two torque flow paths through the gearing, thereby causing torque delivery to drop momentarily at the automatic transmission output shaft. This condition, which can be referred to as a “torque hole,” occurs before the off-going clutch disengages. A large torque hole can be perceived by a vehicle occupant as an unpleasant shift shock. The preparatory phase for the synchronous shift is hereafter defined as a time period prior to the torque phase. The inertia phase for the synchronous shift is hereafter defined as a time period when the off-going clutch starts to slip, following the torque phase.
In the case of a non-synchronous automatic transmission, the upshifting event involves engagement control of only an on-coming friction element, while a companion clutching component, typically a one-way coupling, automatically disengages to reduce both speed ratio and torque ratio. The non-synchronous upshift event can be divided into three phases, which may be referred to as a preparatory phase, a torque phase and an inertia phase. The torque phase for the non-synchronous shift is hereafter defined as a time period when the on-coming clutch torque is purposely raised for its engagement until the one-way coupling starts slipping or overrunning. This definition differs from that for the synchronous shift because the non-synchronous shift does not involve active control of the one-way coupling or the off-going friction element. According to a conventional upshift control, during the torque phase of the upshifting event for a non-synchronous automatic transmission, the torque transmitted through the oncoming clutch increases as it begins to engage. A kinematic structure of a non-synchronous upshift automatic transmission is designed in such a way that torque transmitted through the one-way coupling automatically decreases in response to increasing oncoming clutch torque. As a result of this interaction, the automatic transmission output shaft torque drops during the torque phase, which again creates a so-called “torque hole.” Before the one-way coupling disengages, as in the case previously described, a large torque hole can be perceived by a vehicle occupant as an unpleasant shift shock. The preparatory phase for the non-synchronous upshift is hereafter defined as a time period prior to the torque phase. The inertia phase for the non-synchronous upshift is hereafter defined as a time period when the one-way coupling starts to slip, following the torque phase.
U.S. Pat. No. 4,724,723 discloses one method for eliminating a so-called “torque hole.” That method assumes, however, that output shaft torque can be measured and used in executing a control algorithrm. In the case of the design of the '723 patent, engine throttle position is increased during a preparatory phase of the non-synchronous shift event. Simultaneously, engine spark timing is retarded based on an output torque measurement, which cancels the effects of the throttle change and maintains constant engine torque. The on-coming friction element torque remains zero during the preparatory phase, unlike the control of the present invention, as will be explained. During the torque phase of the system of the '723 patent, spark timing is restored to increase engine torque. This engine torque increase, again, is based on output torque measurements while the on-coming friction element torque starts increasing from zero value or a non-significant level and the off-going friction element torque is reduced toward zero value or a non-significant level during the torque phase.
The inertia phase of the control system of the '723 patent begins when the off-going friction element starts to slip, following the torque phase. During the inertia phase, the engine throttle position and engine spark timing are controlled to reduce engine torque. During shifting, the output shaft torque remains relatively constant.
Thus, according to the '723 patent, engine spark timing control, which is based on measured transmission output shaft torque, is actively used to maintain a constant engine torque during the preparatory stage while the engine throttle increases. Then it relies upon engine spark timing control based on output shaft torque measurements during the torque phase to raise engine torque.